1. Field of the Invention
The present invention relates generally to a load cell for use in a weighing apparatus such as an electronic scale and more particularly, to the load cell for electrically detecting a load imposed on a strain inducing element of a specific configuration, by the utilization of an electric characteristic of a strain detecting element such as, for example, a strain gauge, mounted on the strain inducing element.
2. Description of the Prior Art
The load cell used in an electronic scale generally is employed in the form of a strain inducing element as shown in FIG. 19 and comprises fixed and movable rigid bodies a1 and a2 opposite to each other and a pair of parallel beams b1 and b2 extending between the fixed and movable rigid bodies a1 and a2. All of those elements a1, a2, b1 and b2 are defined by forming a cavity c in a generally rectangular cubic metal block so that the fixed and movable rigid bodies a1 and a2 occupy respective opposite end portions of the metal block while the parallel beams b1 and b2 occupy respective opposite side portions of the metal block. The cave c is so shaped and so configured as to leave pairs of thin walled strain generating areas d1 and d2, d3 and d4, the strain generating areas d1 and d2 of one pair being defined at respective junctions of opposite ends of the beam b1 with the fixed and movable rigid bodies a1 and a2 while the strain generating areas d3 and d4 of the other pair are defined at respective junctions of opposite ends of the beam b2 with the fixed and movable rigid bodies a1 and a2.
The prior art strain inducing element A is so designed and so structured that, when a load W is applied to the movable rigid body a2, a considerable deformation occurs at the thin-walled strain generating areas d1 to d4 as shown by the phantom line, causing the movable rigid body a2 to undergo a parallel downward motion relative to the fixed rigid body a1. In other words, the strain generating areas d1 to d4 generate respective strains in a quantity proportional to the applied load W to reduce any possible adverse influence brought about by a bending moment. In practice, the fixed rigid body a1 of the strain inducing element A is rigidly secured to a base E of the electronic scale while the movable rigid body a2 is coupled with a scale table F. Strain detecting elements G such as, for example, strain gauges, are immovably placed on respective surfaces of the strain inducing element A in alignment with at least two of the strain generating areas d1 and d2 (or d3 and d4) so that a tensile strain and a compressive strain induced in the strain generating areas d1 and d2 as a function of the load placed on the scale table F can be detected by the respective strain detecting elements G thereby to provide an indication of the load W.
According to the prior art, the mounting of the strain detecting elements to the strain inducing element is carried out by bonding the strain detecting elements comprising a film base made of synthetic resin having a strain detecting resistance circuit formed thereon. Since the strain detecting elements is flexible and susceptible to damages, not only is a mechanical handling thereof difficult to achieve, but also a complicated and laborious job is required to mount the strain detecting elements to the strain inducing element. In addition, a substantial length of time is required up until the bonding agent used is cured or hardened, posing a problem associated with workability.
On the other hand, the Japanese Patent Publication No. 62-59767, published in 1987, discloses the formation of a patterned strain detecting circuit directly on the strain inducing element by the utilization of a thin-film technology and a photolithographic technique. The use of the thin-film technology in which a vapor deposition process is performed requires the strain inducing element to be loaded into a vacuum chamber and, therefore, the number of the strain inducing elements produced per unitary time is limited by the size of the vacuum chamber, posing a problem associated with the mass productivity.